Altering Object.prototype.name breaks in the REPL

[nem035]

• Version: Tested on all from 5.11.1 to 7.6.0.
• Platform: Darwin 15.6.0 Darwin Kernel Version 15.6.0: Thu Sep 1 15:01:16 PDT 2016; root:xnu-3248.60.11~2/RELEASE_X86_64 x86_64
• Subsystem: Util

Problematic code:

Object.prototype.name = "bug"
Object.prototype

Sample run:

Code breaks in stylizeWithColor the following line.

Why?

Because we are printing a property, the call to stylizeWithColor passes 'name' as the styleType where 'name' style type is actually ignored.

These style types are stored in the plain object inspect.styles so the execution expects inspect.styles['name'] on the first line of stylizeWithColor to return undefined (since it is ignored) and not enter the if statement.

function stylizeWithColor(str, styleType) {
  var style = inspect.styles[styleType]; // <-- this should return `undefined`

  if (style) { // <-- this shouldn't happen
    return `\u001b[${inspect.colors[style][0]}m${str}` +
           `\u001b[${inspect.colors[style][1]}m`;
  } else {
    return str;
  }
}

However, since we added a "name" property on Object.prototype, inspect.styles['name'] doesn't find 'name' directly but does find it up the [[Prototype]] chain and actually returns the string"bug".

function stylizeWithColor(str, styleType) {
  var style = inspect.styles[styleType]; // <-- this returns "bug"` because it found it up the [[Prototype]] chain

  if (style) { // <-- // <-- this happens because "bug" is truthy
    return `\u001b[${inspect.colors[style][0]}m${str}` +
           `\u001b[${inspect.colors[style][1]}m`;
  } else {
    return str;
  }
}

This means that the code tries to execute inspect.colors["bug"][0], which gives undefined[0], and throws the error in the issue.

A funny consequence is that, if we make name equal to a color value used in inspect.colors, then the error wouldn't happen and the property name will be printed in that color.

Proposed solution

Use an object not linked to Object.prototype for inspect.style.

const styles = {
  'special': 'cyan',
  'number': 'yellow',
  'boolean': 'yellow',
  'undefined': 'grey',
  'null': 'bold',
  'string': 'green',
  'symbol': 'green',
  'date': 'magenta',
  // "name": intentionally not styling
  'regexp': 'red'
};
inspect.styles = Object.assign(Object.create(null), styles);

If confirmed, I'd be happy to implement it.

---

Inspired by this StackOverflow question

[addaleax]

If confirmed, I'd be happy to implement it.

I personally wouldn’t have a problem with that, a prototypeless object is certainly not wrong for styles.
But tbh, if you’re messing with Object.prototype, you’re on thin ice anyway. 😄

[nem035]

@addaleax cool. And you're definitely right, but I guess the REPL would be the best place to experiment with such ice skating 😄

[nem035]

Although not directly related to this bug, it might also make sense to use a prototypeless object for inspect.colors. What do you think @addaleax?